1. Field of the Invention
This invention relates to a powdered binder for binding refractory particles such as sand used for making a mold and a process for preparing a mold by using such a binder.
2. Description of the Prior Art
In practical molds for casting metals, there have been used inorganic binder materials such as sodium silicate and organic ones such as phenol resins, furan resins, etc. All the molds prepared by using these binder materials are durable to pressure or heat exerted by a molten metal. However, a mold containing an inorganic binder has such drawbacks that it is very difficult to remove castings therefrom and it requires a lot of man-hours to produce the castings after pouring, because the mold is very difficult to disintegrate. A mold prepared from an organic binder does not cause such problems, but injurious gases are generated when a molten metal is poured thereinto.
In order to harden a mold, both a hardening process using a chemical reaction of binder components incorporated into molding sand, and a process wherein a mold is hardened by blowing CO.sub.2 gas or an amine gas into the mold after a pattern is filled with molding sand containing a binder may be employed. In the case of the former, a period of time during which the molding sand can be effectively used, i.e., a bench life is limited, because the chemical reaction takes place simultaneously with the addition of a binder to the molding sand. The latter is further classified into a so-called CO.sub.2 process wherein the hardening is effected by blowing CO.sub.2 gas into molding sand containing a binder such as sodium silicate, a process wherein the hardening is effected by blowing an amine gas into molding sand containing a hydroxyl group-containing resin (e.g. phenol resin) and a polyisocyanate as binder, and a process wherein a combination of an acrylic copolymer or a phenol resin and calcium hydroxide is hardened by CO.sub.2 gas. The mold production efficiency of these processes is satisfactory, because the bench life of molding sand is longer than that utilizing a chemical reaction of binder components contained in molding sand, and a mold is rapidly hardened as a gas is introduced. However, the conventional CO.sub.2 process has a defect that a mold is difficult to disintegrate, because sodium silicate is used as a binder. Also, the process using an amine gas suffers from the toxicity and unpleasant odor of such gas. Further, the process wherein a mold comprising a copolymer of an acrylate ester, ammonium acrylate and sodium acrylate and calcium hydroxide is hardened by blowing CO.sub.2 gas thereinto has a drawback that not only the working environment is polluted by ammonia gas generated during the preparation of molding sand or the formation of a mold, but also the strength of the mold immediately after CO.sub.2 blowing is unsatisfactory. Moreover, the process using a phenol resin provides a mold whose strength is lowered, because the mold is hardened only by natural drying.
As disclosed in our U.S. Pat. No. 4,269,256, we already proposed, as a technique for solving the aforesaid problems associated with the CO.sub.2 process and the other prior art processes for producing a mold, a method for preparing a mold which comprises mixing refractory particles with a binder material comprising a combination of an alkali-neutralized product of at least one copolymer selected from .alpha.-olefin-maleic anhydride copolymers, styrene-maleic anhydride copolymers and methyl vinyl ether-maleic anhydride copolymers with at least one of polyvalent metal hydroxides and oxides, and then hardening the mixture by CO.sub.2 gas.
According to the present invention, it has become possible that while retaining the advantages of the above-described process, the problems associated therewith are alleviated, resulting in that the method can be widely used, the molds prepared thereby are more easily tractable, and economical loss is minimized.
The above-described prior art included the following problem. That is, the operation of preparing a solution of an alkali-neutralized product of a copolymer to be used was not simple, and an alkali-neutralized product, i.e., a liquid binder dissolved in an alkali solution was used in order to maintain the binding efficiency of a copolymer.
Thus, it has been found difficult to choose a binder material having the binding property suitable for the properties of a material to be bound or the purpose of using a bound product, as the concentration of a polymer in solution is fixed.
A polymer solution is generally prepared to have concentration which is most frequently needed. However, a solution of a fixed concentration is inconvenient, when finer adjustment of the concentration is required for the purpose of using it in such case that it is necessary to increase the amount of a polymer without changing the amount of water. For such purpose, it is required to provide many solutions having different polymer concentrations. However, it is substantially disadvantageous and not practicable to provide many solutions of different polymer concentrations.
When a polymer is used as a solution, it is difficult and sometimes practically impossible to disperse a small amount of a solution of a high concentration throughout particles having a high surface area.
Though it may be considered to be a simple operation to change a polymer into a solution, it actually takes a long time, and the preparation of a solution having a desired concentration for each case is disadvantageous. Thus, there is a drawback that the extent of application is limited in the use of a polymer solution having a fixed concentration as a binder for particles.
Next, a polymer solution is not easily tractable. Such solution has generally a high viscosity, and in particular, its viscosity further increases when powders such as an alkaline earth metal hydroxide and the like are suspended therein. Accordingly, the amount of the solution sticked to a container, a metering unit and the adjuvant devices therefor is large and causes a significant economical loss. Further, precise metering cannot be expected in the case of a simple procedure. Further, a polymer solution comprises at least 50% of water. A loss caused by dealing with the everywhere-available water during the storage and transfer of containers, and the like is not a little.
Further, when an alkaline earth metal hydroxide powder such as calcium hydroxide is suspended in a polymer solution for the purpose of convenience, calcium hydroxide is precipitated at a certain concentration of the polymer practically applicable in a wide extent of use. Accordingly, continuous agitation is required in order to use the mixture at a correct ratio of the components. This is a troublesome operation. In order to avoid such problem, a suitable amount of a styrene-butadiene rubber latex may be added to prevent the precipitation of calcium hydroxide. This styrene-butadiene rubber latex can be also used in the present invention as an adjuvant to increase the strength of binding. However, though a styrene-butadiene rubber latex increases the strength of binding and prevents the precipitation of calcium hydroxide, it produces unpleasant odor upon heating and pollutes the working environment. Water-soluble powders of a polymer not in the state of a solution are now commercially available, but they can be scarcely used, because they are ammonia-neutralized products and significantly pollute the environment by generating ammonia gas upon the incorporation of calcium hydroxide and heating. Further, the generation of ammonia gas causes a blowhole in castings, when the casting mold contains a certain kind of binder material. Thus, it is often avoided to use a material capable of generating ammonia gas.